Electron emitting filaments have been employed for many years in electron discharge devices, such as ion sources, electron discharge tubes and the like. Most commonly, such filaments have been made of tungsten. However, to achieve adequate emission of electrons, tungsten filaments must be heated to very high temperatures, in excess of 3000.degree. C., close to the melting point of tungsten at approximately 3410.degree. C. At such high operating temperatures, the vapor pressure and evaporation rate of tungsten are relatively high, so that the useful life of tungsten filaments, operated at such temperatures, is correspondingly short. Even at a temperature of 3000.degree. C., the emission density of tungsten is rather modest, about 15 amp/cm.sup.2.
Indirectly heated boride cathodes have been known for a number of years, in the form of metal hexaborides, having the general formula MB.sub.6, in which the metal M may be an alkaline-earth metal, a rare-earth metal, or thorium, all of which form interstitial compounds of such general formula, having the same crystal structure. One such compound is lanthanum hexaboride which is known for its high electron emission per unit area at temperatures far below its melting point. The relatively low operating temperature is associated with low vapor pressure and low evaporation rate, resulting in long operating life under conditions of continuous operation.
While indirectly heated boride cathodes have been known for a number of years, such cathodes have not been used in practical commercial electron discharge tubes, because the boron attacks the base metal to which the boride coating is applied and forms interstitial boron alloys with the base metal. When this occurs, the boron framework around the alkaline-earth or rare-earth metal collapses and permits the boron to evaporate. With tantalum as the base metal, this effect is least pronounced, and it is very slow with rhenium and is nonexistent with graphite.
Indirectly heated cathodes made of lanthanum hexaboride have other drawbacks. They are electrically "noisy" in that they have non-uniform emission, due to difficulties in making the lanthanum hexaboride adhere to the supporting heater, with the result that the heating of the cathode material is non-uniform.